This section illustrates useful background information without admission of any technique described herein representative of the state of the art.
Rock is gained from the earth for crushing by exploding or excavating. The rock can also be natural rock and gravel or construction waste. Both mobile crushers and stationary crushing applications are used for crushing. An excavator or wheeled loader loads the material to be crushed to a feed hopper of the crusher, from where the material to be crushed may fall in a chute of a crusher or a feeder moves the rock material towards the crusher.
In a gyratory or cone crushers, mineral material is crushed by moving an inner blade (crushing cone) with relation to a fixed outer blade. The inner and outer blade define therebetween a crushing chamber. As known, the gyratory and cone crushers are adjusted for different production demands by changing the profile of the crushing chamber, the extent of acentric movement of the crushing cone i.e. the stroke, the rotation speed of the crushing cone and the setting of the crusher.
It is attempted to make full use of the crushing capacity of a gyratory or cone crusher so that the crusher is being loaded continuously with a great crushing power and at the same time the used crushing power is directed to produce desired product distribution. Interruptions in the crushing process reduce efficiency.
Material may accumulate between the inner blade i.e. wear part and an arm of a crusher body, the support cone and slide ring, or output hopper and the support cone and cause an undesired condition referred to as bridging in which the material carries the support cone from underneath. In this description, bridging is not used to deal with another meaning of bridging in which the feed opening of the crusher becomes blocked by objects large enough to enter the crushing chamber but unable to do so, because getting stuck sideways in the feed opening or they block the entry from each other and the feed becomes prevented. The bridging of a gyratory or cone crusher that forms underneath the support cone is yet different in that the bridging may start carrying the support cone.
Bridging forming between a support cone of a gyratory or cone crusher and an underlying arm may be caused by impure feed (e.g. a rag, reinforcing bar, tree stump) accumulating onto an underlying arm of the support cone so that bridging begins to form with the material that is being crushed and the bridging starts to carry the support cone. The bridging may be caused by an oil leak, to which bigger stones get stuck with dust and sand and ultimately a bridge is forms onto the arm and carries the support cone. When the bridging material starts to carry the support cone, an upper thrust bearing that supports the support cone disconnects from the pressure plate and a bearer failure occurs. The bridged material may cause fast wearing and breaking of a dust seal of the crushing cone and in that way fast contamination of oils by dirt. The bridging typically resides on only one side of the main shaft bending the support cone or main shaft askew and also the cam bearing of the crusher may become damaged. For preventing bridging a wiper is arranged in some crushers in connection with the support arm to wipe the arms. The wiper may have short life, because it may, for example, break away and it is also exposed to wearing caused by heavy abrasion. Additionally, the operator of the crusher may overly trust in the presence of the wiper and thus not necessary recognize beginning of bridging before damaging of the crusher.
An objective of the invention is to detect bridging of a gyratory or cone crusher especially during crushing. An objective of the invention is to pre-inhibit possible damage caused by bridging. An objective of the invention is to improve usability and efficiency of a crusher.